While the current electronic elements and products have been designed to meet the requirements for multifunction, high operating speed, high operating power and miniaturized volume, the high-density electronic elements in the electronic products with high operating power tend to produce more high-temperature heat during operation thereof. Therefore, heat dissipating mechanisms must be used to timely dissipate the produced heat for the circuits in the electronic products to maintain normal operation. Among others, the cooling fan is one of the most widely employed heat dissipating mechanisms. Airflows produced by the cooling fan enable increased heat exchange rate to achieve the effect of quick heat dissipation.
Generally, for the purpose of controlling the cooling fan to rotate at a stable speed, a pulse-width-modulation (PWM) to direct current (DC) voltage circuit is used for such control.
Please refer to FIGS. 1A and 1B. A conventional PWM to DC voltage circuit is generally a single-order electronic switch resistor-capacitor (RC) integrating circuit, which consists of a transistor Q, a capacitor C, and five resistors R. When the state of an input PWM signal is OFF, that is, the PWM signal is “low” or zero (0), the PWM signal would not be able to trigger the transistor Q. At this point, the transistor Q is OFF, and electric current from a reference power supply VCC flows through the resistors R to the capacitor C until the capacitor C is fully charged and becomes saturated, as indicated by the waveform T in FIG. 1B.
When the state of the input PWM signal is ON, that is, the PWM signal is “high” or one (1), the transistor Q is immediately triggered by the PWM signal. At this point, the transistor Q is ON, allowing the electric current from the reference power supply VCC to quickly flow to a ground GND and be consumed. Meanwhile, voltage stored in the capacitor C is quickly discharged until electric charges at two ends of the capacitor C are completely discharged, as indicated by the waveform T in FIG. 1B.
Therefore, when the RC integrating circuit is controlled to charge or discharge through triggering of the transistor Q by the PWM signal, a DC level can be output to a control unit (not shown), such as a fan driving integrated circuit (IC), or a fan micro control unit (MCU), so that the control unit determines the fan rotating speed output based on the received DC level and controls the fan operation.
When the transistor Q is triggered by an input PWM signal having a relatively high frequency, such as 100 KHz, to control the RC integrating circuit to charge and discharge, the obtained DC level output can be in a steady state without apparent fluctuation. However, when the transistor Q is triggered by an input PWM signal having a relatively low frequency, such as lower than 100 Hz, to control the RC integrating circuit to charge and discharge, the obtained DC level output has relatively apparent fluctuation. Therefore, when trying to acquire the DC level to determine the fan rotating speed, the control unit would not be able to accurately acquire the DC level due to the relatively large fluctuation in the DC level. As a result, the fan rotating speed output is floating and unstable.
In brief, the conventional PWM to DC voltage circuit has the following disadvantages:                1. The single-order RC integrating circuit has fixed charge and discharge path, which could not be effectively regulated.        2. In the case of a low-frequency PWM signal input, the control unit fails to accurately acquire the DC level, resulting in unstable fan rotating speed output.        
It is therefore tried by the inventor to develop an improved PWM DC steady-state output circuit to overcome the drawbacks in the conventional PWM to DC voltage circuit.